This invention relates to two cycle multi-cylinder engine and more particularly to an improved porting arrangement for such engines.
As is well known, two cycle engines are very popular because their ported nature makes them very simple. In addition, the firing of the cylinder for each revolution increases the specific output of the engine. However, there are a number of problems in connection with the utilization of ported engines.
One of the major problems deals with the fact that the intake cycle takes place at the same time and substantially overlaps the exhaust cycle. In fact, the intake cycle is utilized to purge the exhaust gases from the cylinder through a process that is commonly referred to as "scavenging."
However, when the scavenging is employed in an engine, there is a risk that the fresh air charge may also pass out of the exhaust port with some of the exhaust gases. This problem is particularly troublesome if fuel is also mixed with the exhausted mixture before it has had an opportunity to burn.
One popular type of scavenging system employed with two cycle engines is the Schnurle type. With Schnurle type scavenging, one or more scavenge ports are placed in proximity to the exhaust port. The flow of air into the combustion chamber from the scavenge ports is directed toward the opposing side of the cylinder wall for redirection upwardly and across the cylinder head. The charge then flows back downwardly to the exhaust port. This type of scavenging also uses, at times, an auxiliary scavenging port which is directly opposed to the exhaust port. Although this type of scavenging is very effective, there nevertheless is some concern that the fresh charge may pass out of the exhaust port.
A scavenging type of system has been proposed that employs a supplemental scavenge or tumble port that is disposed transversely to the main scavenge ports. This port introduces a tumble flow into the cylinder on the side facing away from the exhaust port. This permits the attainment of stratification and also improves or reduces the likelihood that fuel will pass out of the exhaust port. A construction of this type is shown in U.S. Pat. No. 5,671,703, issued Sep. 30, 1997 and assigned to the assignee hereof.
Although the system shown in that patent is very effective, there still seem to be ways to further improve performance. For example, it has been found that the utilization of tumble, although helpful is not always desirable. There is, however, desire to at least redirect the scavenge flow from the main scavenge ports so that the charge is directed somewhat away from the side opposite to the exhaust port.
It is, therefore, a principal object of this invention to provide an improved scavenging system for a two cycle engine.
It is a still further object of this invention to provide an improved scavenging system for an engine that achieves good scavenging and also which will permit stratification and ensure against fuel from passing out of the exhaust port.
Where an engine is provided with a porting arrangement that includes scavenging ports on the side of the exhaust port and also a supplemental, scavenging port that forms a primary function of redirecting the flow from the main scavenge ports, additional problems arise. That is, that the ports are generally positioned in such an area that if a multiple cylinder engine is provided, the main scavenging passages for adjacent cylinders must be directly adjacent each other in the plane containing the cylinder bore axes. Thus the cylinder bores must be spaced axially from each other so that there is clearance between these passages. This results in a longer than desired engine.
It is, therefore, a still further object of this invention to provide an improved scavenging system that uses main scavenging ports and a supplemental port for assisting in stratification and wherein the ports are configured so that an inline engine can be compact in length.
It has also been found that a scavenging system as described can be very useful in achieving stratification if fuel is injected into the stream of air circulated from the supplemental scavenging port. In fact, in some instances it may be desirable to inject the fuel into the scavenging passage serving this port. Since the exhaust port is positioned on an outer side of the cylinder block, this makes the positioning of the fuel injector difficult.
It is, therefore, yet another object of this invention to provide an improved scavenging system for an engine of the described type wherein a fuel injector can be easily placed for servicing and still spray into the supplemental scavenging flow for stratification purposes.